Regardless of the rule about about the non-removable built-in modestly layer, I think the main takeaway is the part about 'no nude child avatars'.
If you're wearing your own opaque modesty layer, removable or not, what are LL actually going to ban you for? How will they even know it's non-removable?
And what exactly is another user even reporting you for? Wearing the wrong kind of underpants?
HOWEVER, it's probably also important to make sure your avatar doesn't have any accentuated contours in the areas you're covering up. That might get you in trouble.
From a screen picture, no one could tell if the modesty layer was removable. However, LL can examine your Current Outfit folder to determine what your avatar is currently wearing. (For that matter, there are scripts that can list what your avatar is wearing, though an ordinary user would not be able to determine if your underwear was covering your modesty layer or was the modesty layer without finding a picture or actually acquiring the skin and underwear/tattoo.)
As for reporting "wrong kind of underwear", I would expect that creators will make their modesty layers look exactly like the pictures provided by LL as the safest way to demonstrate compliance with the policy.
And since the policy requires child and child-like avatars only wear "proper child bodies or skins", yes, wearing the wrong kind of underwear will soon be a ban-able violation.
And the policy does also state that implying the presence of genitalia is also prohibited.
As for why the requirement to make the modesty layer part of the skin or body, that avoids the possibility of the child avatar being accidentally nude. If the layer is part of the skin/body texture, then a missing inventory item can't cause the compositor to produce a nude avatar.